The invention pertains to a system and method for generating an AC power output signal from a DC power input signal. More specifically, the invention pertains to a DC-to-AC power converter which is well suited for starting AC motors.
DC-to-AC power converters are often used in uninterruptible power supplies. This is because one common method of storing back-up power is through the use of rechargeable batteries, which store and disburse power as DC energy. Correspondingly, when the main power source fails and the uninterruptible power supply provides back-up AC power from the battery supply, the stored DC energy needs to be converted into AC power.
Furthermore DC energy is often stored in the uninterruptible supply at voltage levels below the typical peak-to-peak line voltage levels of the main power supply. Correspondingly, when stored DC energy is converted into AC power the voltage level needs to be boosted or stepped up. One or more transformers are commonly used for boosting or stepping up the voltage level. At least one technique allows for varying the winding ratio of the primary and secondary windings for adjusting the voltage levels and/or the current levels.
In many instances uninterruptible power supplies provide back-up power for motors, one example being sump pump motors. During the start up time of a single phase motor, the torque required to initiate shaft rotation of the motor must exceed the torque demanded by the motor load. This causes the current in the motor winding to increase, in many instances, by a factor of 6 or 7 times the motor""s rated current. Generally, this current exists for between 15 and 20 sixty hertz cycles, or between 250 ms and 333 ms. For power supplied by the local utility, this typically has no effect on the power quality.
However for switch mode power supplies, consistent with those used in uninterruptible power supplies, a large current draw from a motor starting can result in immediate shut down or damage to the power supply. Often drawing excessive current can result in the magnetics involved in the power conversion process to become magnetically saturated, causing them to lose inductance, thereby rendering them ineffective. Since most uninterruptible power supplies involve the use of transformers of some sort, which electrically isolate the input from the output, when the magnetics become saturated the output voltage falls to zero almost immediately.
Additional detrimental effects are further possible. When the core of the transformer has saturated, all of the magnetizing inductance is eliminated from the primary winding. As the output voltage decreases, due to the current draw of the motor, the control circuitry in the uninterruptible supply will typically respond by increasing the duty cycle or on time of the magnetics. However as the duty cycle increases and the inductance in the primary winding falls to zero, the current through the primary winding which is approximated by the equation IL=(VLxc3x97TON)/L becomes very large and potentially damaging to the other circuit elements, like switching semiconductors.
Correspondingly, there is a continuing need for a DC-to-AC power converter circuit which is capable of handling large currents associated with starting a motor.
A system for generating an AC power output signal from a DC power input signal incorporates a DC-to-DC boost regulator including a plurality of boost stages coupled in series for receiving a DC input signal and producing a boosted DC power signal. The system further incorporates a DC-to-AC converter, coupled to the DC-to-DC boost regulator, for receiving the boosted DC power signal and generating an AC power output signal. A control element is coupled to both the DC-to-DC boost regulator and the DC-to-AC converter.
In one aspect of the invention, each of the plurality of boost stages includes push-pull drivers, a transformer and a charge storage element. The turning on and turning off of the drivers is controlled by the control element. When a respective driver is turned on, current and voltage are coupled via the transformer, rectified and charge accumulates on the charge storage element.
Where the load is an AC motor to be started, the required, starting current saturates the transformer(s) and the output peak-to-peak AC voltage begins to fall. Circuitry coupled to the secondary of the transformer(s) provides a minimum DC output voltage of a value on the order of the amplitude of the input battery voltage. The output AC peak-to-peak voltage, as a result, will drop only to twice the battery voltage.
As the motor begins to start, the load current diminishes and the transformer(s) come out of saturation. Once the transformer(s) come out of saturation, they begin functioning normally, boosted DC voltage increases and the peak-to-peak AC output voltage also increases toward its rated output value. As the motor speeds up, the required current will drop back to its steady-state nominal operating value.
In another aspect of the invention, the DC-to-AC converter includes a first pair of switches coupled in series between the boosted DC power signal and ground and having a first output terminal coupled between the first pair of switches. The DC-to-AC converter further includes a second pair of switches coupled in series between the boosted DC power signal and ground and having a second output terminal coupled between the second pair of switches.
The control element independently activates the switches in both the first pair of switches and the second pair of switches for producing an AC power output signal across the first and second terminal. Conversion processing can be carried out by pre-stored, executable instructions.
A method for generating an AC power output signal from a DC power input signal incorporates the steps of receiving a DC power input signal, boosting the DC power input signal for providing a boosted DC power signal. The boosted DC power signal can then be converted to an AC power output signal.
In at least one aspect of the invention the system and method for generating an AC power output signal from a DC power input signal are used in connection with an uninterruptible power supply.
These and other aspects and attributes of the present invention will become increasingly clear from the following detailed description of the invention and the embodiments thereof, from the claims, and from the accompanying drawings.